Switching regulators are designed to provide a regulated output voltage from an unregulated input voltage. They are frequently implemented in battery powered electronic devices to regulate the battery output voltage which, when charged or discharged, can be greater than, less than, or substantially the same as the desired output voltage.
In general, a switching regulator works by periodically transferring small amounts of energy from the input voltage source to the output. This is accomplished with the help of one or more power switches and a controller which regulates the rate at which energy is transferred to the output. FIG. 1 illustrates a buck switching regulator 100 that works in this general manner to step-down an input voltage VIN to provide a regulated output voltage VOUT. Buck switching regulator 100 includes a power switch module 110, a low-pass filter 120, a sampling circuit 130, and a duty cycle controller 140.
In the step-down regulator of FIG. 1, the basic circuit operation is to close switch 150 for a time tON and then open it for a time tOFF. The total on and off time of switch 150 is referred to as the switching period T. Switch 160 is controlled in the opposite manner as switch 150 and is off while switch 150 is on, and on while switch 150 is off. Thus, ignoring any voltage drop across switches 150 and 160, the voltage at the input to filter 120 is VIN during the time tON and ground or zero during the time tOFF.
With switches 150 and 160 turning on and off, high-frequency voltage pulses are applied at the input of low pass filter 120 and an averaged DC level comes out as VOUT. By altering the ratio of the on time of switch 150 to the switching period, the averaged DC level of VOUT can be changed.
Duty cycle controller 140 is configured to adjust the ratio of the on time of switch 150 to the switching period in accordance with a feedback signal provided by sampling circuit 130. The feedback signal is related to the difference between VOUT and a reference voltage equal to a desired value of VOUT. The ratio of the on time of switch 150 to the switching period is altered as needed by duty cycle controller 140 to regulate the output voltage VOUT at the desired level.
There are several different topologies for implementing duty cycle controller 140. Depending on the topology, the ratio of the on time of switch 150 to the switching period (i.e., the duty cycle) can be altered in a number of ways. The two most common approaches are pulse-width modulation (PWM) and variable frequency. In PWM based control topologies, the switching period is fixed and the on time of switch 150 is varied. Conversely, in variable frequency control topologies, the switching period is not fixed and changes as the on time and/or off time of switch 150 is varied.
Hysteretic switching regulators are one type of switching regulator based on a variable frequency control topology. These switching regulators have several advantages over switching regulators based on PWM control topologies. For example, unlike switching regulators based on PWM control topologies, hysteretic switching regulators do not require an oscillator and therefore are generally simpler to implement.
However, the potentially wide switching frequency variation of hysteretic switching regulators during operation makes it difficult and expensive to filter electromagnetic interference (EMI) caused by the switching action, which can interfere with sensitive RF components near and powered by the regulator.
Therefore, what is needed is a hysteretic switching regulator with reduced switching frequency variation.
The present invention will be described with reference to the accompanying drawings. The drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number.